Background/Purpose:  A major manifestation of Hemophilia A, an X-linked bleeding disorder, is hemophilic arthropathy (HA), a debilitating degenerative joint disease that is caused by intraarticular bleeding. HA typically begins with haemophilic synovitis (HS), hypertrophy of synoviocytes with inflammation of the synovium and a neovascular response, followed by joint erosion and, ultimately, arthropathy with cartilage destruction and erosion of the underlying bone. HS has features in common with inflammatory arthritides such as rheumatoid arthritis (RA). The pro-inflammatory TNFα is a major target for treatment of RA. TNFα is synthesized as a membrane-anchored precursor that is released by the TNFα convertase (TACE, also referred to as ADAM17). We have recently uncovered a crucial role for TACE and its regulator, inactive Rhomboid family member 2 (iRhom2), in the pathogenesis of inflammatory arthritis in mice. Since RA is caused, at least in part, by inappropriate release of TNFα, we hypothesize that iRhom2/TACE/TNFα also have pivotal roles in promoting HA.

Methods: To determine whether blood cells induce TNFα shedding, we exposed macrophages from WT and iRhom2-/- mice to intact and lysed red blood cells (RBC) and measured TNFα in supernatants. We used a joint puncture model in Fviii-/-, Fviii-/-Tnfa-/-, Fviii-/-iRhom2-/- and Fviii-/- etanercept-treated mice to simulate intra-articular bleeding. TNFα in the joint was measured on day 2 after puncture, and histological analyses and microCT were performed at 2 weeks.

Results:  Treatment of WT macrophages with intact and lysed RBC in vitro activated TNFα shedding, but not in macrophages lacking iRhom2. In vivo studies of Fviii-/- mice subjected to knee joint puncture showed severe hemarthrosis and high elevated levels of TNFα on day 2 and synovial invasion with enhanced neovascularization in the joint space and cortical thickening of bone on day 14 compared to WT. MicroCT analysis showed a significant decrease in trabecular bone volume (-75%±12, n=8 mice), number of trabeculae (-40%%±10), and a significant increase in trabecular separation (+71%%±35) in the punctured knee compared to the contralateral non-puncture knee. Inactivation of TNFa in Fviii-/- mice using TNFα-/-, iRhom2-/- and etanercept-treatment dramatically and significantly (p<0.01, n=7-9 per group) reduced the osteopenia in the HA/HS model and improved bone trabecular parameters compared in Fviii-/- mice. TRAP staining demonstrated that the bone loss in Fviii-/- mice was mainly caused by a local increase in osteoclast number (+132%±63) and osteoclast surface per bone surface which was abrogated in Fviii-/-Tnfα-/-, Fviii-/-iRhom2-/- and Fviii-/- etanercept-treated mice. Immunohistochemistry studies showed that although all of mice developed synovitis, the number of macrophages in the synovial membrane was markedly reduced in Fviii-/-Tnfα-/- and Fviii-/-iRhom2-/- compared to Fviii-/-.

Conclusion:  Our results support the hypothesis that the iRhom2/ADAM17/TNFα signaling pathway contributes to the pathogenesis of HA/HS and the associated osteopenia observed in HA/HS patients and suggest that this pathway as a target for treatment.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/inactive-rhomboid-family-member-2tnf%ce%b1-convertasetnf%ce%b1-pathway-is-essential-to-the-pathogenesis-of-haemophilic-arthropathy/